Macrocyclization nitrogen heterocycles

The transition metal-catalyzed reductive carbonylation of o-substituted nitrobenzenes is a useful method for the synthesis of nitrogen heterocycles. Application of this methodology to the preparation of l,4-dihydro-2/f-3,l-benzoxazin-2-one derivatives and to the synthesis of the 3,1-benzox-azepin-2-one (42) has been reported <93JOM(451)157>. Reaction of 2-(2-nitrophenyl)ethanol (41) (100% conversion) with carbon monoxide in the presence of a palladium(Il) catalyst gave (42) in 60% yield, together with some of the macrocyclic dimer (43) ( nation (2)). It is suggested that a nitrene complex may not be involved with the palladium catalyst in view of the reaction path selectivity observed <93JOM(45i)l57>. 

A different approach to dendritic sensors involves modification of a sensor core unit with dendritic substituents to confer beneficial solubility properties. An example of a sensor core unit is the porphyrin macrocycle, a heterocycle that has been employed extensively in prototypical photochemical sensor systems. Vinogradov and co-workers have exploited the versatile photoactive porphyrin sensor unit as a fluorescence-based pH indicator for use in biological assays [73], by attaching acid terminated polyamide-ether dendrons as substituents (Figure 8.12). The two imino nitrogen atoms present in the free-base porphyrin are susceptible to stepwise protonation to afford initially a cation and then a dication, respectively. Upon protonation, both the emission and absorption fluorescence spectroscopic characteristics of the porphyrin core are subject to dramatic hypochromic shifts. This spectroscopic phenomenon formed the basis for an accurate pH indicator with potential applications in proton gradient determination studies in biological systems. 

Mamedov VA, Gubaidullin AT, Khafizova EA, Samigullina AI, Bauer I, Habicher WD (2014) Macrocycles containing nitrogen heterocycles 11. Calixarene t3q>e macrocycles based on m-bis(imidazo[l,5-a]quinoxalin-4(57f)-on-l-yl)benzene for the encapsulation of organic guest molecules. Chem Heterocyel Compd 50(2) 237-245... 

Fu et al. revealed that Cul/L-proline-catalyzed intramolecular cyclization of amides 136 could proceed smoothly to afford medium- and large-sized nitrogen heterocycles 137 (Scheme 49) [87], The introduction of a phosphoryl or tert-butoxycarbonyl group at iV-termini was found essential for this macrocyclization. 

Ag(ll) can be stabilized by nitrogen-containing heterocycles such as py, polypyridyls and macrocycles. In fact, Ag(I) disproportionates in the presence of the 14-membered tetraaza macrocycle meso-Me5[14]aneN4(M)... 

Nudeobases have pyrimidine and purine (a combination of pyrimidine and imidazole) heterocyclic nuclei which have two ligating nitrogens placed in angular directions. The angular dispositions of pyrimidine, imidazole, and 1,7-purine nitrogens respectively being 120°, 150°, and 90° impart to these bases, heteroatom-induced endocyclic donor ability towards metals which drive self-assembly processes to form macrocyclic systems. The metallacalixarenes formed from nudeobases and various metal salts are discussed with respect to each base. 

Figure 10-2. Although they are relatively rare in biology, organometallic compounds with metal-carbon bonds are important. An typical example is coenzyme B12. The cobalt is co-ordinated to a tetraazamacrocyclic ligand, and has one axial nitrogen donor heterocyclic ligand and one axial alkyl group. The figure illustrates only the core of the macrocycle and the axial groups.

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